There are a few possibilities, one, it is two caps in one can, unlikely in a 70's era amp or the third pin is an extra connection point to keep a heavy part in place. It is probably connected to the neg pin or nothing. Have a look at the bottom of the circuit board to see if two pins share a leed or remove it and check with an ohm meter. The third possibility is the third pin connects to the outside shell of the cap, that would connect to ground. In any case, unless it is two caps you should be able to replace it with a two leed cap and given the age of the amp, not a bad idea...John
